In the market for magnetic recording and reproducing apparatus, improvement in the recording density exceeding 40% per year has been demanded and it is estimated that the recording density will reach Tbit (terabit)/in2 (square inch) about in the year of 2011 in accordance with the growing rate at present. For the terabit class magnetic recording apparatus, increase in power and resolution performance is inevitable in magnetic recording and reproducing heads.
In the current magnetic recording and reproducing apparatus, a CPP-GMR (Current Perpendicular to Plane Giant Magnetic Resistance) head and a TMR (Tunneling Magneto Resistance) head that flow a sense current perpendicular to stacking surface is proposed as a constituent element thereof. The spin valve type reproducing apparatus described above use a magnetic body (free layer) as a detection method of a leak magnetic field from a medium and it shows resistance change with respect to a relative magnetization direction to a magnetic body magnetically pinned in one direction (pinned layer).
For improving the resolution power of the current CPP-GMR head and the TMR head, it is necessary to reduce the thickness of constituent films. Particularly, as the bit length decreases, a gap width Gw should be narrower in order to obtain a high resolution power. For example, Gw=18 nm in a reproducing head to a medium at 2 Tbit/in2 and the total thickness for the device constituent films should be 18 nm or less.
Then, for a hard disk having a terabit class surface recording density, an external magnetic field sensor having a feature of high resolution power and a high output is necessary. As the external magnetic sensor having the high resolution power and with low noises, a reproducing head applying a spin accumulation effect has been proposed (for example, in Patent Literature 1).
The spin accumulation effect is a phenomenon that when a current flows from a ferromagnetic body to a non-magnetic metal, spin polarized electrons are accumulated in the non-magnetic metal within a range of a spin diffusion length. The spin diffusion length represents a distance in which spin information is lost (spin is inversed), and this is a value inherent to a substance.
The effect is attributable to that when a current flows from a ferromagnetic body to a non-magnetic metal, spin polarized electrons (spin electrons) are injected and the chemical potential is different between the up spin electrons and down spin electrons since a ferromagnetic body generally has a different spin density at the Fermi level (number of electrons is different between the up spin electrons and down spin electrons). It has been known that the non-magnetic conductor behaves ferromagnetically within a spin diffusion length due to the accumulated spin electrons (for example, refer to Non Patent Literatures 1, 2)
The reproducing head that utilizes the effect uses a spin current-injected magnetic body as a pinned layer and the other magnetic body as a free layer opposing to a recording medium. Due the spin accumulation effect, since the pinned layer and the free layer can be disposed at positions spaced apart within a range of the diffusion length (distance in which spin information can be transmitted), the inter-shield distance (gap length) opposing to the recording medium can be narrowed. Further, since current does not flow directly to the sensing portion of the free layer, there is a possibility capable of decreasing Johnson noises, etc.
The output voltage difference ΔV due to the spin accumulation effect utilizing magnetic tunneling conduction is represented by the following formula (Non Patent Literature 3).
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                Δ          ⁢                                          ⁢          V                =                                                            P                2                            ⁢                              λ                N                                                                    A                N                            ⁢                              σ                N                                              ⁢                      exp            ⁡                          (                              -                                  d                                      λ                    N                                                              )                                                          (        1        )            where    P: spin polarization rate,    λN: spin diffusion length of an non-magnetic body,    d: distance between two magnetic bodies,    AN: a cross sectional area of a non-magnetic body,    σN: electric conductivity of the non-magnetic body.
In accordance with the equation, for improving the output voltage, 1. improvement in the spin polarization rate, 2. increase in the spin diffusion length, and 3. refinement of a device are important factors. Among them, improvement in the spin polarization rate is an important factor for the increase in the output voltage difference.